With the spread of the Internet in recent years, the use form of the computer has been changed, e.g., to the form of processing a great volume of moving picture data and sound data with a personal computer. Along with these trends, the storage capacity required of the magnetic recording media, such as hard discs, has increased.
In a hard disc apparatus, a magnetic head slightly floats from the surface of a magnetic disc with the rotation of the magnetic disc and magnetic recording is performed by non-contact recording. This mechanism prevents the magnetic disc from breaking by the contact of the magnetic head and the magnetic disc. With the increase of density of magnetic recording, the floating height of a magnetic head is gradually decreased, and now the floating height of from 10 to 20 nm has been realized by the use a magnetic disc comprising a specularly polished hyper-smooth glass substrate having provided thereon a magnetic recording layer (a magnetic layer). In a magnetic recording medium, the layer structure of a CoPtCr series magnetic layer/a Cr under layer is generally used, and the direction of easy magnetization of the CoPtCr series magnetic layer is controlled in the direction of in-plane of the film by the Cr under layer by increasing the temperature in forming the magnetic layer and the under layer as high as 200 to 500° C. Further, the magnetic domain in the magnetic layer is segregated by accelerating the segregation of Cr in the CoPtCr series magnetic layer. Areal recording density and recording capacity of hard disc drive have markedly increased during the past few years by technological innovation, e.g., the floating height reduction of a head, the improvement of the structure of a head, and the improvement of the recording film of a disc.
With the increase of throughput of digital data, there arises a need of moving a high capacity data, such as moving data, by recording on a commutable medium. However, since the substrate of a hard disc is made of a hard material and the distance between a head and a disc is extremely narrow as described above, there is the fear of happening of accident by the collision of a head and a disc and entraining dusts during operation when a hard disc is used as a commutable medium such as a flexible disc and a rewritable optical disc, and so a hard disc cannot be used.
Further, when a high temperature sputtering film-forming method is used in manufacturing a magnetic recording medium, not only productivity is poor but the cost in mass production increases, thus hard discs cannot be manufactured inexpensively.
On the other hand, the substrate of a flexible disc comprises a flexible polymer film and is excellent in commutability, since it is a medium capable of contact recording, and so flexible discs can be manufactured inexpensively. However, commercially available flexible discs of nowadays have such a structure that the recording layer is formed by coating a magnetic powder on a polymer film together with a polymer binder and an abrasive. Therefore, the high density recording characteristics of the magnetic layer of flexible discs are inferior to those of hard discs having a magnetic layer formed by sputtering, and the achieved recording density of flexible discs is only 1/10 or less of that of hard discs.
Accordingly, a ferromagnetic metal thin film type flexible disc having a recording film (a magnetic layer) formed by the sate sputtering method as in hard discs is suggested. However, when the same magnetic layer as that of hard discs is tried to be formed on a polymer film, the polymer film is greatly damaged by heat and it is difficult to put such a flexible disc to practical use. Further, since the contact of a head with a medium is inevitable, a hard protective layer is indispensable. Therefore, it is also suggested to use highly heat resisting polyimide and aromatic polyamide films as polymer films, but these heat resisting films are very expensive and it is also difficult to put them to practical use. When a magnetic layer is tried to be formed with cooling so as not to give thermal damage to the polymer films, the magnetic characteristics of the magnetic layer are insufficient, thus recording density can be hardly improved.
On the other hand, it has come to be known that when a ferromagnetic metal thin film comprising a ferromagnetic metal alloy and a nonmagnetic oxide is used, almost the same magnetic characteristics as those of the CoPtCr series magnetic layer formed under a high temperature condition of from 200 to 500° C. can be obtained even when a recording layer is formed under room temperature. As such a ferromagnetic metal thin film comprising a ferromagnetic metal alloy and a nonmagnetic oxide, ferromagnaticmetal thin films having a so-called granular structure which is proposed in hard discs can be used (refer to, e.g., JP-A-5-73880 and JP-A-7-311929).
However, even when a magnetic layer having such a granular structure is formed at room temperature, since heat by sputtering is applied, the gas contained in the polymer film support is released, so that the crystal growth of the under layer and the magnetic layer extremely lower as shown in FIG. 2. Therefore, the control of crystal orientation is very difficult, so that satisfactory characteristics have not been obtained yet.
On the other hand, in direct read after write and rewritable optical discs represented by DVD-R/RW, the head and the disc are not close to each other as in a magnetic disc, and so they are excellent in commutability and widespread. However, from the thickness of light pickup and economical viewpoints, it is difficult for the optical disc to take such a disc structure that both surfaces can be used as recording surfaces as in a magnetic disc which is advantageous to improve capacity. In addition, the optical discs are low in areal recording density and in data transfer speed as compared with a magnetic disc, so that their performance cannot be said to be sufficient to be used as rewritable type high capacity recording media.